Swedish Patent Application No. 9103522-0 describes an internally located device for sound suppression of a channel system, particularly for an exhaust system of internal combustion engines.
A channel system that is connected to a disturbance sound source, for instance an intake system for internal combustion engines, generally consists of an air filter housing as well as a channel positioned before and after the air filter housing. The channel disposed after the air filter housing has its other end connected to the inlet manifold of the engine so that the channel disposed after the air filter housing constitutes a channel for conveying the filter purified air to the engine. In this type of system, the cross sectional area of the inlet manifold is considerably larger than that of the air channel. The above-described system has, from an acoustic point of view, an equivalent in an exhaust system. That is, there is to be found at least one channel or tube, both ends of which are coupled to volumes or spaces constituting silencers. The last channel of the exhaust system as seen in the flow direction has one end coupled to the environment or outside atmosphere which constitutes an infinite volume.
For an inlet system or an exhaust system constructed according to the above description, so-called standing sound waves arise in the channel between the two volumes or spaces. At these resonance frequencies, the so-called insertion damping is very low, or sometimes even negative. That is, pulse sounds from valve openings pass out through the system with very low sound suppression--or sometimes even as an amplified sound.
The first standing sound wave, the so-called "lambda half" (.lambda./2), has its maximum sound pressure midway between both channel ends and its maximum velocity at both respective ends. Multiples of .lambda./2, for example .lambda. or 1.5.lambda., have 2 and 3 sound pressure maxima respectively between the ends of the channel. One of these velocity maxima is located at the respective channel ends. The frequency of these standing waves is determined by the channel length and the gas temperature. The gas temperature dependence means that the frequency, i.e., the resonance amplifications, varies strongly between a hot system and a cold system, which is the case, for example, in an exhaust system.
In U.S. Pat. Nos. 3,396,812 and 3,415,338, so-called quarter wave pipes are used to reduce standing waves in an exhaust system. These solution alternatives have the common drawback that the temperature in quarter wave pipes normally differs significantly from the temperature in the exhaust system channel which may vary between ambient temperature in a cold engine to 600.degree.-700.degree. C. at full load. This means that the constant length of the quarter wave pipes corresponds to any of .lambda./2, .lambda., 1.5.lambda., etc. in the exhaust channel only within a very limited exhaust gas temperature range.
Since the so-called quarter wave pipe of traditional form has a very narrow band suppression characteristic, the solutions mentioned above also exhibit great limitations in connection with inlet systems in which the temperature variations are considerably lower. For a 2 liter petrol engine, the flow velocity in the filter channel between the filter housing and the inlet manifold is up to about 25 m/s at full load and 5000 rpm. During motor braking, i.e., a closed throttle condition, the flow velocity is almost 0 m/s. The flow differences between about 0 m/s and 25 m/s implies that the so-called acoustic impedance in the region of the inlet to the quarter wave pipe coupled to the system channel varies significantly. The narrow band characteristic of the quarter wave pipe in combination with variations in its inlet impedance means that maximum frequency adaption must be made very carefully and even in spite of this, cannot be optimized for all cases of motor operation.
Further, great drawbacks in traditional forms of quarter wave pipes are their side band affects. That is, if optimum adaptation/suppression has been achieved for example for .lambda./2, interference inevitably is obtained above as well as below the frequency which corresponds to .lambda./2. For instance, if the cross sectional area of the quarter wave pipe is equal to that of the channel where .lambda./2 arises, the amplifications are obtained by a known method at about 0.7 and 1.4 times the original resonance frequency respectively. If, for example in 5-cylinder engines, sound suppression is wished at a standing wave caused by a second multiple of the ignition frequencies, as a result of the side band effect a strong amplification is obtained instead of the third multiple of the ignition frequency. This occurs at the same number of revolutions as the original problem.